Journal of Magnetics 21(3), 374-378 (2016) http://dx.doi.org/10.4283/JMAG.2016.21.3.374

© 2016 Journal of Magnetics

A Study on the Transformer Design considering the Inrush Current Reduction

in the Arc Welding Machine

In-Gun Kim1, Huai-Cong Liu1, Su-Yeon Cho2, and Ju Lee1*

1Department of Electrical Engineering Hanyang University, Seoul 04763, Korea2Korea Automotive Technology Institute(KATECH), Cheon-an 31214, Korea

(Received 9 March 2016, Received in final form 21 April 2016, Accepted 27 April 2016)

The transformer used in an inverter type arc welding machine is designed to use high frequency in order to

reduce its size and cost. Also, selecting core materials that fit frequency is important because core loss increases

in a high frequency band. An inrush current can occur in the primary coil of transformer during arc welding

and this inrush current can cause IGBT, the switching element, to burn out. The transformer design was

carried out in AP method and amorphous core was used to reduce the size of transformer. In addition, sheet coil

was used for primary winding and secondary winding coil considering the skin effect. This paper designed the

transformer core with an air gap to prevent IGBT burnout due to the inrush current during welding and

proposed the optimum air gap length.

Keywords : arc welding machine, ferrite & amorphous core, inrush current, air gap core

1. Introduction

Recent inverter arc welding machine systems can have

high speed switching by using high performance switch-

ing elements and are controlled in high frequency. Power

IC and circuit, transformer, and reactor for switching take

up the largest volume among the components of arc

welding machine. Therefore, since the capacity of trans-

former and reactor is inversely proportional to frequency,

it is necessary to operate them in high frequency [1, 2].

Since the capacity of transformer is influenced by

magnetic flux density value and the magnetic flux density

is determined by core materials, selecting appropriate

materials is important. In general, the core materials that

are usable in a high frequency band are ferrite, amorph-

ous. When they have the same capacity, the differences of

two materials are magnetic flux density, production cost,

core loss, etc. This paper designed the transformer based

on the information mentioned above.

1) The basic design of transformer was carried out

using the AP method.

2) The secondary winding coil was formed in a parallel

circuit as it has low voltage and high current.

3) PSP coil and sheet coil were used for coil method

considering the skin effect and proximity effect.

4) To minimize the size of transformer core, it was

made in different materials (ferrite and amorphous) and

then compared.

Due to the nature of welding machine, when the trans-

former core is saturated after the welding rod is in touch

with a base metal, then a high current above 1000A flows

in the primary winding of transformer. This current is

called inrush current and the power IC can be burned out

due to the inrush current. Thus, it is important to choose

the core size of transformer considering the inrush current

during design. The methods to prevent inrush current are

as follows.

1) Make the size of core big so that the transformer is

not saturated.

2) Maintain the size of transformer core as it is and

increase the number of the turns of coil instead.

This paper uses an air gap to prevent the saturation of

core while maintaining the size and number of turns of

core [3, 4].

2. Transformer Design Method for Welding Machine

2.1. Area Product Design

AP design method is one of the methods of transformer

©The Korean Magnetics Society. All rights reserved.

*Corresponding author: Tel: +82-2-2220-0342

Fax: +82-2-2295-7111, e-mail: julee@hanyang.ac.kr

ISSN (Print) 1226-1750ISSN (Online) 2233-6656

Journal of Magnetics, Vol. 21, No. 3, September 2016 − 375 −

design and is expressed by the multiplication of Wa and

Ac. AP method can be expressed in formula 1, formula 2,

and Fig. 1. The standard for determining the area of Wadepends on the core shape. AP design method can be

shown as in Fig. 1 [5-7].

(1)

(2)

The Pt is a total power, Bm is the residual magnetic flux,

Kf is a waveform coefficient, Ku is a window utilization

factor, f is the switching frequency, J is a current density.

The maximum window size of AP method must be made

considering the space factor of coil, so that there is no

interference between each other when the coil is winded

inside the window. AP is proportional to Ac and is

inversely proportional to f, Bm, and J. That is, the volume

of transformer core gets smaller when it is designed in

high frequency and magnetic flux density [8]. Since the

switching current through the IGBT flows in the

transformer primary, AP must be designed with about 10%

margin [9]. Figure 2 shows the inverter arc welding

machine system.

2.2. Parallel Circuit

The winding method using parallel circuit is as in Fig.

3. X represents the start point of coil and X’ represents

the end point of coil. In the primary size, AA’ and BB’

are connected in series. The CD of secondary winding

coil is coiled in the forward direction and EF is coiled in

the reverse direction. Also, coil C is connected to F and D

is connected to E, thereby making a total of 2 parallel

circuits. C’D’E’F’ are connected together. Since the

secondary winding coil has low voltage and high current

(77V, 389A), the current is divided into two through 2

parallel circuits and thus the current density is halved

[10].

2.3. PSP Winding and the Sheet Coil

PSP winding method and sheet coil are used to reduce

P a cA W A= ×

4( 10 ) / ( )P t m f uA P B f J K K= × × × × ×

Fig. 1. AP Design Method.

Fig. 2. (Color online) Overview of Inverter Arc Welding Machine System.

Fig. 3. Winding Method using Parallel Circuit.

− 376 − A Study on the Transformer Design considering the Inrush Current Reduction in the Arc Welding Machine…

− In-Gun Kim et al.

the influence of proximity effect and skin effect.

(3)

μ is a permeability of the coil, σ is a conductivity of the

coil.

The skin depth δ of coil from formula 3 is 0.554 mm,

and a coil much thinner than skin depth must be used

(Primary coil is 0.2 mm, Secondary coil is 0.5 mm). In

addition, in the high frequency band by the proximity

effect, the size and status of current changes greatly

depending on the location of the two parallel coils of

secondary winding. Therefore, the parallel coil of secondary

winding must be interleaved with the primary coil and

have the same locations. Such placed parallel coil have

the same magnetic flux and magnetic field distribution

around it and prevents current unbalance [11]. As a result,

PSP and SPS coil, not PS or SP coil, must be used. Figure

4 shows the PSP coil method and Fig. 5 shows the

specifications of transformer designing.

The two coil methods in Fig. 6 represents the layer coil

and sheet coil, and both methods have the same current

density. The two models operate for an hour and their

temperature saturation was confirmed. Copperplate coil

and the coil method of PSP model were used based on the

result of Fig. 6.

2.4. Comparison according to Core Materials

To minimize the optimum of the same allowable fre-

δ = 1

πfμσ-----------------

Fig. 4. PSP Winding Method and Sheet Coil.

Fig. 5. (Color online) Specifications of Transformer Designing.

Fig. 6. (Color online) Temperature Saturation Curve in the Layer Coil and Sheet Coil Model.

Fig. 7. (Color online) Transformer made by using Ferrite and

Amorphous Core.

Journal of Magnetics, Vol. 21, No. 3, September 2016 − 377 −

quency, magnetic flux density needs to be higher. There-

fore, the transformer was made with ferrite and amorphous

core that can be used in a 14.4 kHz section. The specifi-

cations of each transformer are as in Table 1 and Fig. 7.

Since the amorphous has a magnetic flux density that is

three times higher than that of ferrite as in Table 1, it has

lower weight and number of turns of coil within the same

frequency. Amorphous is better in terms of performance,

but it requires more precision in production than ferrite

because multilayered core is made by molding powder

due to the nature of amorphous production. The two

models were applied inside the arc welding machine for

the experiment and the output was satisfying. The

experiment environment is as in Fig. 8.

3. Air Gap Core Method

A very high current flows inside a welding machine as

its switch repeats shortening and opening. The output

current of welding machine is as in Fig. 9. The time must

be more than 6 ms since shortened to satisfy the rated

current at 600A during the load.

The inrush current of transformer primary winding

occurs when the load is connected to the secondary wind-

ing of transformer and the switch is on. During the rated

load of welding machine, welding works normally after 5

ms. The maximum welding time must be within the

allowable current of the switching element that the inrush

current of primary winding can endure. If the maximum

welding time is exceeded, the switching element is burned

Table 1. Specifications of the Ferrite and Amorphous Core of

Transformer.

Value

Ferrite

Core TR

(PM7)

Amorphous

Core TR

(AMCC-250)

Core Weight (kg) 3.48 2.23

Coil Weight (kg) 3.79 2.47

Turn Ratio 32:4 24:3

Flux Density (T) 0.11 0.45

Switch Freq. (kHz) 14.4 14.4

Core Loss (W) 56.84 57.32

Coil Loss (W) 20.22 10.11

Efficiency (%) 97.74 96.96

Fig. 8. (Color online) Experiment System of Arc Welding

Machine.

Fig. 9. (Color online) Output Voltage and Current Waveform

during Load.

Fig. 10. (Color online) Comparison of Magnetic Flux Satura-

tion according to Air Gap.

− 378 − A Study on the Transformer Design considering the Inrush Current Reduction in the Arc Welding Machine…

− In-Gun Kim et al.

out [12, 13]. An air gap (0-5 mm) was given to increase

the maximum welding time. The inrush current of primary

winding is decreased as the induced voltage becomes

smaller by primary winding, since the inductance in the

core drops when the air gap increases. However, if the air

gap exceeds 5 mm, the average magnetic flux density

(amorphous 0.45T) decreases and cannot satisfy the rated

output during load. The result of finite element analysis

on the differences of air gap are as in Fig. 10.

The highest current that can flow in the used IGBT is

−600~600A. The characteristics of the inrush current of

primary winding are as in Fig. 11. This waveform shows

welding after the welding time was set up to 20 ms and

switch open was made.

When the time gets above 12.5 ms, the allowable current

of IGBT is exceeded regardless of air gap. Thus, the

maximum welding time cannot go beyond 12.5 ms in the

used IGBT. The model without an air gap exceeds −600A

at 10 ms of welding time, while a model with 5 mm-air

gap does not exceed −600A at 10 ms of welding time.

This enables welding at up to 10 ms. During welding, the

output at load as well as the allowable current of IGBT

and maximum welding time must be considered.

4. Conclusion

Since an inverter arc welding machine uses high fre-

quency band, it has an advantage of reducing the size of

transformer. It is necessary to reduce the size of trans-

former core to save cost. However, when the size of core

is decreased, magnetic flux is saturated and huge inrush

current occurs by the primary winding of transformer.

The huge inrush current of primary winding can make

IGBT, the switching element, burn out. As a result, since

the maximum welding time is proportional to the time of

the safe operation of IGBT, the transformer was designed

by inserting an airgap to decrease the inrush current of

primary welding. It is necessary to appropriately choose

the maximum length of air gap as it influences the output

of load.

Acknowledgment

This work was supported by the Human Resources

Program in Energy Technology of the Korea Institute of

Energy Technology Evaluation and Planning (KETEP),

granted financial resource from the Ministry of Trade, Industry

& Energy, Republic of Korea (No. 20154030200900).

This work was supported by the National Research

Foundation of Korea (NRF) grant funded by the Korea

government (Ministry of Science, ICT & Future Planning)

(No. NRF-2016R1A2A1A05005392).

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Fig. 11. (Color online) Comparison of the Waveform of Pri-

mary Winding Inrush Current according to Air Gap.